Posted
by
samzenpus
on Wednesday December 07, 2011 @03:43PM
from the use-it-or-lose-it dept.

First time accepted submitter ambermichelle writes "GE Hitachi Nuclear Energy has proposed to the U.K. government to build an advanced nuclear reactor that would consume the country's stockpile of radioactive plutonium. The technology called PRISM, or Power Reactor Innovative Small Module, would use the plutonium to generate low-carbon electricity. The U.K. has the world's largest civilian stockpile of plutonium. The size of the stockpile is 87 tons and growing. Nuclear reactors unlock energy by splitting atoms of the material stored in fuel rods. This process is called fission. For fission to be effective, neutrons – the nuclear particles that do the splitting and keep the reaction going – must maintain the right speed. Conventional reactors use water to cool and slow down neutrons, keeping fission effective. But water-cooled reactors leave some 95 percent of the fuel's potential energy untapped."

I am amazed that conventional water-cooled reactors are only 5% efficient. It sure casts the seemingly low efficiency factors of other alternative fuels(such as the cheapest solar panels) into a different light.

Save solar, wind and various methods of deriving hydroelectric power, all electric power generators boil down to the downright caveman primitive method of heating water into steam to drive turbines. No one has yet figured out anything better.

I am amazed that conventional water-cooled reactors are only 5% efficient. It sure casts the seemingly low efficiency factors of other alternative fuels(such as the cheapest solar panels) into a different light.

But you are talking about 5% of the energy from a fuel with an energy density which is about 1,000,000 times the energy density of coal

Nevertheless, it's unbelievable that after 50 years of nuke plants, we've not moved on to more efficient plants or don't do reprocessing on a mass scale.
It's rank stupidity to rip up the earth to extract a fairly rare substance for 5% of its potential and then have to find safe methods to store the 95% for 10,000 years.

It's not the thermodynamical efficiency. The usual water cooledreactors use slow neutrons (water slows the initially fast neutrons fromfission to slower speeds). These reactors can only extract a fraction of availableenergy from the fuel. Liquid metal cooled reactors use heavy metalatoms (sodium, eutectic lead/bismuth) as primary coolant which does not slowneutrons. The fast neutrons are used in fast breeder reactors, which can burnthe fuel more thoroughly or create new fuel (U-239/Pu) as they run.

It's not 5% efficiency. Of the thermal energy they produce, in fact, more of it can be used than coal, since nuclear reactors can operate at higher temperatures than coal furnaces. However, if someone came up with a coal fuel cell, perhaps it could be even more efficient, since it wouldn't lose energy to thermalization. Muscles are not heat engines, they are like 95% efficient.

Only 5% of the nuclei that can be fissioned are. In a different reactor, more of the fuel could be fissioned; with current react

And the energy density of a chunk of Uranium is still orders of magnitude higher than any other practical fuel source. Also, it's not as if we couldn't refine and recycle the fuel and squeeze more of the energy out of it later. Today's nuclear waste storage is tomorrow's 'unlimited energy source'.

I don't know if "efficiency" is the right word... This implies that 95% of the energy available is wasted as heat which can never be used again.

That isn't the case with traditional light water reactors - they don't waste much more heat than any other heat-engine-based plant - however, 95% of the potential energy in their fuel simply can't be used in the first place!

I believe one of the statistics of the Integral Fast Reactor project was that the United States' existing spent fuel stockpiles would be able t

It's not that it's not that efficient, it's that it really doesn't need to be. The energy from fission is mostly captured (although you are dumping a lot of heat), but crucially it leaves high energy products behind in the fuel. It's what makes the spent fuel so hazardous to deal with, which is why it's crazy to suggest burying it in the ground!

Why bury something that has so much juicy energy still in it that we can extract with current technology? The answer is political, of course.

The other factor to consider is the sheer magnitude of the energy we're talking about here. E = mc^2 is not just a handy soundbite.

Why bury something that has so much juicy energy still in it that we can extract with current technology? The answer is political, of course.

The UK used to lead the world with nuclear power. We had more of it than anyone in the 60s, but then we started trying to build new advanced designs which turned out to be harder than we thought. By the time they were coming online in the early 80s the Conservative government was busy selling off all energy infrastructure, but no-one would buy the nuclear bits because of the enormous costs and enormous liabilities.

So now when someone comes along with a fantastic new technology that will solve all our proble

Who said anything about new technology? We have the technology we need right now to reprocess and use spent fuel from PWRs in breeders and other types of reactors, but it's politically sensitive to build reactors that can be purposed to make Pu for weapons if desired. No need for any new pie in the sky technology.

Failsafe meaning that in the case of a catastrophic failure there is no meltdown. Though you asked who is talking about new technology and that's who does it, people trying to gloss over the problems of the state of proven art.

We don't need to invent anything new, just refine well-understood designs and processes.

That is deploying new technology as well, with all the potential problems that entails.

PRISM is a commercialized version of the Integral Fast Reactor (IFR). I personally would rather that plutonium be used for LFTR start charges than used in a big tank of liquid sodium. But since we taxpayers have already spent $35 billion on the IFR and related tech since 1965, it would be nice to get some use out of all that money, even of it is GE that benefits. Just hope there are no major sodium leaks.

But water-cooled reactors leave some 95 percent of the fuel's potential energy untapped."

I gather the problem is that decay products poison the fuel after it's been run for a while. One would still need to reprocess fuel rods on a regular basis. But once that's done, you can get more than 5% of the energy from a fuel rod.

Not with light water reactors you can't. You run out of fissile material - natural uranium is more than 99% U-238, which isn't fissile. Some of this is converted to Pu-239 by neutron capture, but light water reactors only have a conversion ratio of 0.5 or so. For every 10 uranium atoms fissioned, you only get 5 plutonium ones. So at best you can only double the fuel utilisation, and in practice less.

The point of fast reactors was that the conversion ratio can be over 1, so there's no net consumption of fiss

If Iran gets the bomb, I hope for all Iranians its government won't be stupid enough to use it (in any way, like running a test explosion somewhere).

And if any country would feel the urge to stop Iran from obtaining a nuclear bomb, let's hope for all our sakes conventional bombs would be used for that job.

In the meanwhile, "The PRISM reactor actually disposes of a great majority of the plutonium as opposed to simply reusing it over again without ever actually ridding the planet of the substance." sounds lik

Technically speaking, anything in the periodic table over Pb208 (Lead) is radioactive. It's just some of these elements have REALLY long half lives. And the longer the half life, the lower the radioactivity...

considering all atoms have some half life and there for are in some way radio active i'd have to say no.. but if you don't consider normal every day stuff radioactive then i'd point you at Plutonium 244

no. but the usual Pu-239 isn't very radioactive, just emits alphas slowly with a very long half life of 24,200 years. That radiation can't even penetrate your skin or go through a piece of paper. Pu-240 is artificial, usually decays by alpha but sometimes spontaneously fissions, it too has long half-life of more than 6500 years. Then there is Pu-238, emits huge amounts of alphas with its short half-life of 88 years, it's used in RTG batteries and also radioisotope heater units. A kilogram of the stuff gives off 500 watts.

I think even CANDU has limitations of energy extraction that prevent it from extracting as much energy from the fuel as a breeder+reprocessing cycle like the IFR (and PRISM seems to be very similar to the IFR).

Yes, CANDU is actually very inefficient - it doesn't extract all the usable fuel as other reactors can.

However, it does have the advantage that it's impossible to have a meltdown - heavy water is a great moderator. In fact, it's required in order to have a reaction - if there's no heavy water, the fuel's inert. And normal water impedes the reaction as well, so if the cooling system leaks, the reaction stops as well.

Plus, the fuel that comes out needs even heavier processing to become weapons grade.

CANDUs have decay heat like any other reactor, so are quite as vulnerable to meltdowns in the event of loss of cooling.

They're also better at making weapons grade plutonium than LWRs thanks to online refuelling - you can irradiate the fuel elements for a short time only to avoid the buildup of heavier isotopes of plutonium.

Right, and TFA is just a page on the manufacturer's web site. I smell the foul aroma of bullshit. After all if it were as good as they make out why are they trying to sell one to the UK and not just building on in the US or Japan? The US doesn't even have a long term storage facility at the moment and spent fuel is building up at reactor sties, and once that lot is burned up they could charge the UK to deal with their waste.

Don't forget about CANDU reactors [ccnr.org]. They use a heavy-water moderator and are able to burn a wide variety of fuels including plutonium, natural uranium, or "spent" fuel from a light-water reactor.

the history of liquid sodium reactors has been a sad one, look up the Fermi #1 unit in Detroit some time. basically the job of keeping the liquid sodium, which is mightily explosive and gets mightily radioactive as a moderator, inside away from air and water is something that hasn't been solved yet. I would not be stumping the countryside trying to site one.

Just 'cuz I was curious, and it has some peripheral bearing on the question - assuming 19.816 gm/cm^3 for the density of Pu (more than lead) and also assuming (since it's the UK) we're talking "tons" = metric tonnes = 1000kg = 10^6 gm -

87 x 10^6 gm / 19.816 gm/cm^3 = 4.39 x 10^6 cm^3 = 4.39 m^3.

4.39 cubic meters is a single cube 1.637 meters on a side (or a little more than 5 feet/side, for us backward Yanks). More or less the size of a smallish SUV, yes?

Of course their Pu isn't, one hopes, stored all in one solid cube, which would probably exceed critical mass by some large factor. But still, it's not a massive physical quantity of material you're talking about here./TSG/

Does anyone have a rough idea of how much electricity could be produced by this type of reactor using the 87 tons plutonium stockpile? Please express in terms of % of annual electricity consumption by Britain, or another unit readable by laymen.

Nearly all of it. THink of it this way: UK has 18-25% of their electricity nukes (basically, 18 reactors). [world-nuclear.org]
Now, not a big deal, it is about the same as America. What is interesting is that they have accumulated all that fuel since 1967 (average appears to be 35 years). And that is less than 5% burned. So, if UK, puts in these reactors, then they will be able to burn the rest of the fuel. How long will it take? Over a 100 years depending on how many reactors they put in.

Basically, not only will it be burning WASTE fuel, but, it is actually cheaper to burn this then to try and handle the 'waste' fuel. Look at WIPP. It is a true cluster. OTH, these can actually be built for a fraction of what it costs to store all that waste but instead makes money.

In addition, because it is modular, these can be added at the sites that are already handling nuke reactors. With this approach, it allows a plant that is already built to handle large power but heading towards closure to switch

You should really read up on the "Integral Fast Reactor" - the S-PRISM this article is about is evolved from the technology developed in the IFR project.

The main potential safety weakness of an IFR is the possibility of sodium leaks leading to a sodium fire (I'm not sure how they manage this risk; it certainly seems like a potentially nasty problem, but I'm sure they've taken some sort of measures to try to prevent that from happening; I hope they are effective).

But, Sodium fires aside, the type of problems you had an Chernobyl, TMI, and Fukushima-Daiichi simply cannot happen in an IFR-style reactor. You can't get supercriticality/runaway fiisson like happened at Chernobyl; you can't get a meltdown; you don't have to worry about steam pressure overwhelming the containment (because water is not used as the coolant, so hence no steam), and you can't get a hydrogen explosion (again, no water in the reactor).

You might get a hydrogen explosion if, somehow, water started mixing with the sodium, as sodium and water will combine to form sodium hydroxide and hydrogen gas, but if they can keep water out of the reactor, then no hydrogen explosions.

So far as I know, there have only been a few sodium fires amongst all the world's sodium cooled reactors over the last 60 years - the most famous one was in Japan back in the late 90's or early 00's, and while that scared the public, it wasn't actually a disaster - just a relatively minor industrial accident in the end. I've never heard of a sodium fire at a nuclear plant becoming a major problem, so I don't think the risk of sodium fires is actually a big, unmanageable 'ticking time bomb', but again, I'm no expert.

Still, I think the technology looks *very* interesting. Let's face it, we have a nuclear waste problem, and either IFR or another type of fast reactor (such as a molten salt fast reactor) are basically the only way to solve that problem. Let's stop fighting the solution to the nuclear waste problem. It truly is the only realistic solution - burn off that 100,000 year "plutonium problem".

You can't get supercriticality/runaway fiisson like happened at Chernobyl

Fast reactors are somewhat notorious for being trickier to control than (well-designed) thermal ones. It's very difficult to avoid a positive void coefficient, and fairly small changes in the fuel geometry can lead to large changes in reactivity. There was a meltdown in an early FBR caused by thermal expansion causing the fuel to bow inwards, increasing the reactivity. Phenix in France also had unexplained loss of reactivity incidents.

Fast reactors are somewhat notorious for being trickier to control than (well-designed) thermal ones. It's very difficult to avoid a positive void coefficient, and fairly small changes in the fuel geometry can lead to large changes in reactivity. There was a meltdown in an early FBR caused by thermal expansion causing the fuel to bow inwards, increasing the reactivity. Phenix in France also had unexplained loss of reactivity incidents.

The void coefficient is almost unavoidably positive since these reactors

I was wondering why GE was trying to get a new reactor design built in the UK instead of its home country, the US. Then I realised why: our government is the only one that will pay for it. The Conservatives view the government as a way to fund commercial enterprises, to build stuff that no bank would back but which with most of the cost paid for out of taxation are a potential gold mine for the owner. That is the way we build nuclear plants here, the tax payer funds it and takes on most of the risk and clean-up cost while the commercial owner creams off a nice profit during the operational lifetime.

Unfortunately the government always gets ripped off when building anything and the companies running our nuclear facilities seem to be incompetent and unwilling to invest in safety. The plant TFA mentions, Sellafield, is notoriously accident prone, so I'm not sure it is a good idea to give them any more ways to screw up.

Thanks but no thanks GE, get back to us when you have built a working one paid for out of your own pocket.

The Conservatives view the government as a way to fund commercial enterprises

Sounds pretty much like the previous Labour with their enthusiasm for the Private Finance Initiative [wikipedia.org] and such like.

Even if we're "charitable" and assume a "best case" scenario that they were only doing it for cynical accounting and political reasons, to hide costs in the short term (rather than because they wanted to pander to private business interests), the end result is that the scheme was bordering on evil. Labour politicians knew full well that it would work out grossly more expensive in the medium an

Look up "liquid metal embrittlement" to get an idea of the problems the French and Russians have had with large liquid sodium cooled reactors (nobody else has tried). Water doesn't seem like such a bad idea once you know about it. Work is progressing (eg. new Russian reactor) but it's not a solved problem by any stretch of the imagination. However with good design there can be leaks that merely cause downtime instead of catastrophic breaks.Here's a clue - liquid sodium is used for technical and not safety reasons.Whoever is selling you on some snake-oil "sodium is safe" marketing line is not being honest to you and you are making yourself look naive and poorly informed by repeating it.

Let's stop fighting the solution to the nuclear waste problem

Two things, first it only consumes a small portion of nuclear waste and produces a larger volume of a different type of waste - which I'm sure you already know. Second, the established civilian nuclear energy producers have been the ones fighting the solutions to the nuclear waste problem on the basis of cost. I atteneded a seminar on Synrock over twenty years ago and it's only recently that it has been adopted anywhere due to governments pressuring reactor operators to do something with their waste.

Here's a clue - liquid sodium is used for technical and not safety reasons.

That's half true. There's a number of properties that make sodium very attractive as a coolant:

-It is much less corrosive to many steel alloys than is water. Some alloys don't corrode at all.-It allows for a high power density-High thermal conductivity-The reactor need not be pressurised-Low neutron capture cross section-Modest melting point-It only forms short lived radio-isotopes when irradiated-High operating temperature ( as compared to water )

From a safety perspective a properly designed sodium cooled reactor is very unlikely to suffer a LOCA due to the low pressure, natural circulation allows for sufficient heat transport even during a total loss of power, the higher thermal conductivity enables fast thermal feedbacks and the higher thermal efficiency ( due to higher temperatures ) means somewhat less decay heat has to be transported away.

Two things, first it only consumes a small portion of nuclear waste and produces a larger volume of a different type of waste - which I'm sure you already know.

It can completely fission the actinides you feed it, and the waste it produces decay to safe levels within 300 years, as opposed to 100.000 for the original wastes. Plutonium that has been recycled through it would also be almost useless for nuclear weapons since the isotopic composition after 1 or two passes is even worse than reactor grade plutonium. The reason it only consumes a small portion of nuclear waste is because it needs almost 100 times less fuel than a conventional reactor ( thanks to a positive breeding gain ) , which conversely means that if you consider all the waste we have, there's enough fuel for a thousand years or so.

Now there are alternative breeder designs to sodium coolant. Lead, molten salt, helium or supercritical water could all work. They all have their respective advantages and disadvantages.

All power sources are problematic. Energy has a way of making environments uninhabitable to humans... When you start storing large amounts of energy in small spaces things get more dangerous.

But don't let that fool you. Coal seam fires for instance: http://en.wikipedia.org/wiki/Centralia,_Pennsylvania can make an area uninhabitable for decades, centuries...Hydro destroys ecosystems down stream; to some humans, this can destroy their livelihood. And when one damn fails it'll kill hundreds to thousands in a f

Not with modern generators.http://en.wikipedia.org/wiki/Integral_Fast_Reactor [wikipedia.org]Liquid metal thorium reactor are incredibly safe.No event in any nuclear reactor that has ever happened can happen in one. Plus you can burn waste in them.Oh and the waster from these return to background radiation levels in 200-500 year. Very workable, and possible to store on site. No shipping the waste.

The US government should be building 20 of these right now. And the US government should operate them;remove the desire to make bonuses , and all other problems go away with it.

These are the solution until we can get cheaper solar, or maintainable fusion.

The Wikipedia page you linked to mentions that there are no commercial Thorium reactors in existence, only research models. While I agree they would be better than current reactors and help deal with the nuclear waste problem I also agree that the US should be building them, not the UK.

BTW we have cheap solar already. Solar thermal collectors scale, are cheap to build, cheap to run and cheap to decommission, and best of all they work 24/7 and are one of the best available options for meeting peek demand nee

I would hate to see a scaled up Solar Thermal power plant. The largest one that I know of is the SEGS plant in California. As I remember it has a peak power output of ~350MW. But if your talking about 24/7 operation that drops to a small 75MW of output.

To get that 75MW of base load capacity, they have to use 6.5KM^2(I had to look that up ^^) of land. If this technology was scaled up to the size of a nuclear plant that has a base load capacity of 1GW you would be talking about using(some people would say destroying) 90KM^2 land.

Actually, looking at the Invanpah plant which is currently under construction, it's a 392MW(Peak Power) plant that is going to be using ~16KM^2 of land. So the newer plant is even worse on land usage... While it's technically possible to build large solar thermal plants, I don't think your going to find the land to do it. Invanpah was scaled down from initial plans because of land use issues...

I am not so sure about the cost difference either. Invanpah is a 2.2 billion dollar project. When you compute $ per KW of capacity, your looking at about $5,600 per KW. It's hard to find accurate Nuclear plant numbers since so none have been built in the US in 30 years. Looking online I found two numbers on $per/KW a pro nuclear site quoted ~$2000-2500/KW and a anti-nuclear site said ~5000-6000/KW. I am not sure which to believe but even if it's the high number, it lines up with Invanpah cost almost exactly. But the problem is that this is comparing the Peak Power $/KW price of Invanpah vs Nuclear. I looked all over the place and I couldn't the planned capacity factor... but if Invanpah can only generate a base load of ~100-130MW then the cost of Invanpah would be 3-4 times that of the "High" figure vs Nuclear.

Honestly after looking at these numbers I am shocked at just how bad Solar Thermal power really is for baseload generation costs. I didn't think it was good but I never would have thought it was this bad.

I would hate to see a scaled up Solar Thermal power plant. The largest one that I know of is the SEGS plant in California. As I remember it has a peak power output of ~350MW. But if your talking about 24/7 operation that drops to a small 75MW of output.

To get that 75MW of base load capacity, they have to use 6.5KM^2(I had to look that up ^^) of land. If this technology was scaled up to the size of a nuclear plant that has a base load capacity of 1GW you would be talking about using(some people would say destroying) 90KM^2 land.

Your numbers are way off. The older technology used in Spain would need about 575 hectares to generate 1GW, far less than you are claiming. That is old technology too, the newer stuff is more efficient.

I'll grant you that it isn't going to be as compact as a nuclear plant though, but so what? We have plenty of space where no-one wants to live. The EU is looking to north Africa (and now we are best friends with Libya) because 0.3% of the Sahara could power the whole of western Europe. The US has plenty of un

No event in any nuclear reactor that has ever happened can happen in one.

WTF. Where did you get this from? Twenty seconds of research shows the Monju Nuclear Power incident [wikipedia.org] which was a fire caused by a liquid sodium leak. That can obviously repeat in any sodium cooled reactor.

Area under water behind a dam is uninhabitable and unarable, same goes with solar. Wind is just uninhabitable. If you count the amount of land required by them and compare to land made "uninhabitable" by nuclear, average over power generated, nuclear is a clear winner.

The radiation levels in Chernobyl Zone are lower than natural background radiation in some areas around the globe. Year of living in Ramsar in Iran exceeds nuclear industry limits during emergencies! Calling them "uninhabitable" for 1000 years is a bit of an overstatement... Unarable for food production, maybe, but then you can use those areas for production of automotive fuel.

Oh, and don't forget the amount of land made unarable and uninhabitable by heavy metal poisoning from regular industry, just look at mercury pollution in USA.

You might have a point with hydro. That's one renewable that isn't particularly environmentally sound. What you're saying about solar and wind just doesn't make any sense. For one thing, solar isn't just one particular technology. There's solar panels of various kinds, then there's various types of large solar collectors, several types of which are perfectly compatible with farming, for example. Sure you wouldn't want to live in a greenhouse, but there's no reason you couldn't have housing under solar panel

With just about any large system there is potential for a catastrophic failure.

Dams: If the dam fails it could kill hundreds to thousands of people. Likely, no. With terrorist help or just plain stupidity yeah there is a nonzero chance of disaster.Oil spills: These happen much more frequently than nuclear issues and cause significant damage.

Using your logic it would be appropriate to ban planes, cars, trains, etc.

People need to stop letting fear and ignorance rule and actually look at the facts.

It's renewables, not "renewables". You're making an implication that what is commonly called renewable energy is not - that's flat-out wrong. Their primary weaknesses are that they are intermittent and relatively diffuse.
Neither of those has anything to do with the fact that they are "renewable". Understood?

No, you're flat-out wrong. The term 'renewable' indicates an energy source that either never runs out or can be indefinitely replaced. There currently exists no such energy on earth. Solar is no more renewable than any fossile fuel; our great big ball of fire out there is slowly burning out, and when it's done there is absolutely nothing we can do to replace it. We also can't keep renewing it so it doesn't burn out. In fact, if anything fossile fuels are more renewable than solar because it is possible to r

That's silly. You're claiming that the term 'renewable' indicates an energy source of a kind that doesn't exist anywhere in the universe? As for fossil fuels being renewable, if they're renewed, it's from plant and animal matter being fossilized. The animal life gets its energy ultimately from photosynthesizing plant life. Can you tell me where photosynthetic plants get their energy?

A good rule of thumb might be to consider that an energy source that will last for longer than life has existed on Earth and c

Renewables are renewable as long as you don't take into account mining, refining, construction, transportation, and useful lifetime. A solar cell, for example, requires a lot of infrastructure to bring a lot of esoteric elements from around the globe and turn them into something that may generate useful amounts of power for maybe 2 decades (less, if it gets replaced early).

Living doesn't entail a 100% chance of killing or maiming other people, nor does dying. I would say the chances in either event are actually much closer to zero. So yes, your being alive is reasonably safe for me and those around you.

"I came from nothingness, and now I must contemplate returning to nothingness" -- can you prove that?It would seem inconsistent with all the scientific evidence. Or do you know of some event which has no cause?

Today's nuclear plants are ~35% or so thermal efficiency, which is not that bad. Upgrading the generating end to a closed cycle turbine loop and staging multiple loops can raise that efficiency a great deal - 50%+ is realized in some newer natural gas power plants. The nuclear part itself is not the limiting factor.=Smidge=

The higher the temperature of your working fluid, the higher your possible theoretical efficiency can be. The best out there are hitting 60% with a very high temp gas turbine with a steam generator hooked to it's exhaust and a rankine cycle attached to that.

There are some advanced reactor designs that can hit 50% if built, mostly due to higher working temperatures.

What's the problem? Is the sodium radioactive? Because if it isn't, it is no worse than using molten sodium to store solar power. Better even, because the facility isn't on the surface, but buried, and doesn't have to be exposed to the air (allowing for a lot more shielding).
It seems to me that if the sodium comes out underground, you get some magma until the heat dissipates, and that is it. I don't see what the big deal is.

Sorry, but in this universe we have to obey the laws of thermodynamics. There are no 100% efficient electric power generation systems. The efficiency of a real world wind turbine can't be more than 30%. A heat engine (including steam turbine systems or using solar to heat working fluid) can't be more efficient than carnot efficiency limit, less than 42% for real world plants. Direct solar conversion to electricity can't be more than 34% efficient.